As performances of optical elements for IT instruments such, for example, as a digital still camera, digital video camera, a television camera for making a high-definition television and a projector have recently been developed remarkably, various possibilities of optical design have been proposed for improving optical properties of optical instruments which are attributable to refractive index and dispersion of refractive index as represented by chromatic aberration. There is, however, the problem that a sufficient lens element, i.e., optical glass, for realizing such optical design has not been provided by the market yet. Therefore, in the prior art products, a desired optical property has to be sacrificed or, alternatively, such optical property has to be realized by carrying out processing for achieving a complicated shape of lens or by adopting a complex combination of lenses, i.e., increasing the number of lenses used. In any case, such optical design has to be realized at the cost of either optical property or productivity and manufacturing cost.
Particularly, no sufficient optical glass having a refractive index (nd) of 1.79 or over and an Abbe number (νd) in the vicinity of 30 has been provided so far for reasons that (1) such optical glass usually devitrifies, i.e., loses transparency during reheating processing such as reheat pressing, (2) such optical glass generally has poor chemical durability and, as a result, it requires a special care in handling after polishing and (3) the specific gravity of the glass is large and, therefore, weight of a final product is impermissibly large.
Known in the art of the optical glass having a refractive index (nd) of 1.79 or over and an Abbe number (νd) of 27 or over are a SiO2—B2O3—La2O3—Nb2O5—ZrO2—TiO2—RO (R being an alkaline earth metal element) optical glass as disclosed in Japanese Patent Application Laid-open Publication No. Sho 59-50048, a SiO2—TiO2—Nb2O5—R2O (R being an alkali metal element) optical glass as disclosed in Japanese Patent Application Laid-open Publication No. 2002-87841, a SiO2—B2O3—CaO—TiO2—Nb2O5 optical glass as disclosed in Japanese Patent Application Laid-open Publication No. Hei 3-5340, and a SiO2—B2O3—La2O3—TiO2—Nb2O5 optical glass as disclosed in Japanese Patent Application Laid-open Publication No. 2004-18371.
The optical glass disclosed in Japanese Patent Application Laid-open Publication No. Sho 59-50048 satisfies the required optical constants but has such a large specific gravity that weight of the lens becomes large and therefore is not suitable for practical purposes. Moreover, since this optical glass has a strong tendency toward devitrification, when a lens is produced from this optical glass by reheat pressing, special reheating conditions are required. Hence, a lens cannot be produced from this optical glass by reheat pressing but it must be produced by using a grinding and polishing process.
In the optical glass disclosed by Japanese Patent Application Laid-open Publication No. 2002-87841, Abbe number relating to dispersion of refractive index is less than 27. Optical glasses in this region of Abbe number have been supplied in the past but are not sufficient from the standpoint of expanding degree of freedom in the optical design. Moreover, since this optical glass contains a large amount of alkali metal oxide, it has a strong tendency toward devitrification in reheat pressing and, further, there are problems of fusion of the glass to a mold and deterioration in durability of the mold due to solving out of alkali.
Japanese Patent Application Laid-open Publication No. Hei 3-5340 and Japanese Patent Application Laid-open Publication No. 2004-18371 disclose borosilicate lanthanum oxide optical glasses. These optical glasses, however, contain a large amount of boric acid and hence insufficient in chemical durability, particularly in water-proof and acid-proof properties. Care must be taken, therefore, in handling the glass after it has been polished to a desired shape such as a lens or prism with resulting decrease in productivity.
It is, therefore, an object of the present invention to provide, at a small manufacturing cost and a minimum burden to the environment, glass and an optical element having optical constants of a refractive index (nd) of 1.79 or over and an Abbe number (νd) of 27 or over, being free from devitrification and opaqueness in the interior of the glass when the glass is molded by a manufacturing process requiring reheating such as reheat pressing, and having excellent chemical durability.